Most semiconductor wafer processing machines, especially in the later stages of integrated circuit manufacture, work on one wafer at a time, typically holding the wafer on a vacuum chuck. In general, however, lots of approximately 25 to 50 wafers are delivered to the machine at one time, contained in one or two cassettes. Cassettes are magazines having regularly spaced slots, carrying one wafer per slot. It is important to prevent the wafers from contacting each other or other objects because the circuits etched on their upper surfaces are extremely delicate and prone to contamination by small particles.
In modern fabrication facilities, great emphasis is placed on limiting the presence of human operators in the processing area, because humans are the major source of contaminating particles. Thus most process machines are provided with automatic loaders which extract wafers one at a time from the cassettes, move the wafers into position to be engaged by the processing machines, and return the processed wafers to the cassettes, all without human intervention.
The two major operations involved in automatic cassette-to-cassette wafer handling are wafer transport between cassettes and the loader and wafer exchange between the loader and the process machine. The traditional loading mechanism structure includes conveyor belts and elevators dedicated to input and output cassettes: the input cassette elevator lowers the input cassette until the bottom unprocessed wafer is on the input conveyor belt, allows it to be transported out of the cassette onto the belt, and lowers again to bring the next wafer onto the belt. The reverse procedure allows processed wafers to be transported from the output belt into the output cassette. In general, wafer transport between the conveyor belts and the process machine chuck is accomplished by a completely different mechanism, e.g., a rotary shuttle arm. Two drawbacks of this approach are the cost of providing an elevator mechanism for each cassette and the inability to access wafers at random.
An alternative approach that is becoming more popular is the use of robots with appropriate end effectors (e.g., vacuum wands) to extract wafers from the cassettes, place them directly onto the process machine chuck, remove them directly from the process machine chuck after processing, and return them to the same or a different slot of the cassettes. This allows for random access of wafers and eliminates the need for cassette elevators.
Two vacuum wands have been used on opposite ends of a support member that is rotatably mounted on the end of a robot arm so that a processed wafer can be removed from the processing mmachine with one wand and an unprocessed wafer can be immediately dropped off at the processing machine by rotating the other wand into position, without the necessity of another trip to the cassette and back, to increase the time that the processing machines is at work. The robot returns the processed wafer to a cassette, picks up an unprocessed wafer (also doing gross wafer flat alignment adjustments if desired), and can return to the vicinity of the processing machine while the pocessing machine is doing its work and wait there for the next wafer exchange.